Frost Line Depth in St. Clair County, IL: Illinois Codes Range 36

No official local value

No official frost line depth is published for St. Clair County.

No verified code nearby; verified Illinois codes range 36–42 in.

Source and how we checked

We searched this place’s own code and website and found no published frost line depth.

Climate: 94 nights a year at or below 32 °F (NOAA 1991–2020, Belleville Siu Rsch, 5.6 mi).

Checked Oct 2026. Report a correction

Footings Deck Water line Fence post

36–42 inrange of verified Illinois codes
94nights a year at or below 32 °F
86%of nearby soil rated “high” for frost action (USDA)

Does St. Clair County have an official frost line depth?

No adopted frost line depth found for St. Clair County in its official documents (checked Oct 2026). No verified code lies near St. Clair County; the Illinois codes we verified range from 36 to 42 in. Ask the building department for St. Clair County’s own number before you dig.

What frost line depth does St. Clair County use for footings under Table R301.2(1)?

Call 811 (JULIE) at least a few working days before you dig, so buried lines are marked.
Confirm the frost line depth with the St. Clair County building department.
Ask whether your project needs a permit (decks, larger sheds and footings usually do).
Dig below 42 in (the deepest verified state value) unless the building department tells you otherwise.
Book the footing inspection before you pour concrete.

How deep do footings, decks and water lines need to be in St. Clair County?

Drawn with the deepest Illinois official value (42 in). Confirm St. Clair County’s own depth before you dig.

House or garage footing Deck footing Water service line Fence post

House or garage footing in St. Clair County: dig at least 42 in (107 cm). Below the frost line and at least 12 in deep (IRC R403.1.4).

Using the nearby official value of 42 in until St. Clair County confirms its own
Project	Depth	Rule
House or garage footing	42 in	Below the frost line and at least 12 in deep (IRC R403.1.4)
Deck footing (attached)	42 in	Below the frost line and at least 12 in deep (IRC R507.3)
Water service line	48 in	6 in below the frost line and at least 12 in deep (IRC P2603.5)
Fence post bottom	42 in	No code rule; common practice is below the frost line

Work out posts, concrete and footing sizes with the fence post depth calculator or the deck footing calculator.

How cold does it get in St. Clair County?

Average lows are at or below freezing from Dec to Feb. 94 nights a year reach 32 °F or colder; Jan is coldest, with an average low of 24.3 °F.

NOAA 1991–2020 climate normals, Belleville Siu Rsch station, 5.6 mi away, elevation 450 ft.

Monthly temperatures as a table

°F	Jan	Feb	Mar	Apr	May	Jun	Jul	Aug	Sep	Oct	Nov	Dec
Low	24.3	28	36.3	46	56.3	64.5	67.8	65.3	56.9	46.7	36.9	28.3
High	42.9	48.6	59.2	70.6	79.1	87.3	90.2	89.1	83.3	72.6	58.1	46.3

St. Clair County has fewer freezing nights than 68 of the 71 other Illinois places we track. The Illinois median is 127. Coldest: Hoffman Estates; mildest: Madison County.

How likely is the soil in St. Clair County to heave?

Moderate 14%High 86%

USDA rates the sampled soil around St. Clair County as 14% moderate, 86% high for frost action (main soils: Wakeland, Marine, Wakenda).

USDA NRCS soil survey, 9 sample points within 2 mi. Higher frost action means more risk of heave; see frost heave.

St. Clair County at a glance

Frost line depth: No official local value
Population (2023 est.): 251,018
Mean annual temperature: 57.7 °F
ZIP codes (34): 62059 · 62060 · 62201 · 62203 · 62204 · 62205 · 62206 · 62207 · 62208 · 62220 · 62221 · 62223 · 62225 · 62226 · 62232 · 62234 · 62236 · 62239 · 62240 · 62243 · 62254 · 62255 · 62257 · 62258 · 62260 · 62264 · 62265 · 62269 · 62282 · 62285 · 62289 · 62293 · 62294 · 62298